Long known as an interactive-music-film annual gathering that helped launch tech titans Twitter and Foursquare, the SXSW Conference and Festivals this year is jammed with political panels, speeches and documentaries and has become a must-attend venue for 2020 presidential hopefuls.

Scheduled to attend the nine-day Austin conference, which opens Friday: Democratic Sens. Amy Klobuchar, of Minnesota, and Elizabeth Warren, of Massachusetts; former U.S. Housing and Urban Development Secretary Julián Castro; House Minority Leader Kevin McCarthy; and former Colorado Gov. John Hickenlooper. All but McCarthy, a Republican and a supporter of President Donald Trump, have declared an interest in running in next year's presidential election.

Also attending: firebrand Congresswoman Alexandria Ocasio-Cortez and Starbucks CEO Howard Schultz, who stirred controversy earlier this year by announcing he was seriously considering a run for president as an independent.

"We've always tried to showcase what's going to be important in the next few years," said Hugh Forrest, SXSW's chief programmer. "Nothing certainly is more important than the 2020 election."

Now in its 33rd year, SXSW started as a small, local music conference and festival and has mushroomed into one of the biggest and most influential gatherings on the planet, drawing more than 400,000 total attendees and delivering an economic impact of more than $350 million to the region.

More important for politicians is the social-media reach of its army of tech-savvy, smart-phone-toting attendees, said Mark Jones, a political scientist at Rice University in Houston. The attendees' predominant demographic -- left-leaning millennials -- is a key target audience for Democratic candidates hoping to score big in primaries next year. The race kicks off in February, when the Iowa caucuses are expected to mark the first votes of the 2020 presidential election.

Julian Castro, former U.S. Department of Housing and Urban Development (HUD) Secretary and San Antonio Mayor announced he was running for president on Jan. 12, 2019.(Photo: Edward A. Ornelas, Getty Images)

Texas and California are seen as the two top prizes for primary vote-getters on Super Tuesday on March 3, 2020, Jones said.

"It'll be a unique focal point where the left across the United States will be focused on what's going in Austin," he said of this year's SXSW. "You most definitely do no want to be absent."

New to the event this year is a two-day roundtable titled “Conversations About America’s Future,” which will feature some of the the country's top political names, including Klobuchar, Warren and Castro, as well as former Ohio Gov. John Kasich and former Gov. Bill Weld, a Republican from Massachusetts who plans to run against Trump in the Republican primaries next year. Kasich is also considered to be mulling a primary battle against Trump.

SXSW is a platform not only for candidates to get their message across but also to jump start volunteer efforts, Jones said.

"This is one way to reach out to these people and get your name out there," Jones said. "It also gets them on your side on the ground in their home states."

Democratic Senator of Minnesota Amy Klobuchar announces that she is running for President of the United States on Feb. 10, 2019.(Photo: Craig Lassig, EPA-EFE)

SXSW has long welcomed politicians to its events, dating back to former Texas Gov. Ann Richards, who gave a keynote speech at the 1993 conference, Forrest said. Former President Barack Obama held a widely-watched keynote conversation with Evan Smith, chief executive and editor-in-chief of the Texas Tribune, at the conference three years ago. Former vice president -- and potential presidential hopeful next year -- Joe Biden previously attended, as did Ocasio-Cortez and Vermont Sen. Bernie Sanders, who is running for president for a second time in 2020.

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But the increased attention on politics and the sheer number of marquee politicians is unique to this year's conference, Forrest said.

"We’ve seen more politicians each year," he said, "but 2019 will take that to a new level."

Chris Lippincott, an Austin-based political consultant who began attending SXSW in the mid-1990s, said he remembers when the annual event mostly meant catching out-of-town bands in bars along Sixth Street, Austin's entertainment district.

Over the years, he's watched the gathering morph into a tech-driven event, a draw for corporate leaders and, increasingly, a be-seen political meetup.

"SXSW is a lot like Austin: It is constantly growing and changing," Lippincott said.

He added: "Politics has become something we, as a society, cannot help but talk about. In that respect, it's a perfect fit for SXSW."

Even those not on the official agenda this year will make their presence felt. El Paso native and former Rep. Beto O'Rourke, who narrowly lost a Senate race last year to Republican Ted Cruz, will be the subject of a documentary, "Running With Beto," which will screen at the festival.

They'll still be plenty of non-political, traditional SXSW fare, such as panels dissecting digital distrust and discussions on artificial intelligence. Jordan Peele's "Us" — the next film following his blockbuster first effort, "Get Out" — will have its world premiere Friday at SXSW.

Former Republican Speaker of the House John Boehner will attend in his new role as a cannabis-industry lobbyist. He'll be on a panel discussing paths to legalizing marijuana and hurdles the fledgling cannabis industry still faces.

Buttigieg, who has attended SXSW before, said he's not surprised to see so many politicians lining up to attend the conference. The tech world is increasingly tied to the country's economic and political future, he said, and he looks forward to swapping ideas with some of the top political minds in the U.S. That, and scoring some grilled meat from Austin's iconic lunch spot, Franklin's Barbecue, he said.

"Increasingly, what's happening in tech cannot be uncoupled from what's happening in policy and politics," Buttigieg said. "You realize just how central these spheres are to one another’s future."